What is the Periodic Table and Electron Configuration?
The periodic table organizes all 118 known elements by atomic number and electron configuration. Electron configuration—how electrons fill orbitals (1s, 2s, 2p, 3s, 3p, 3d…)—determines element position and chemical properties. Elements in the same group (column) behave similarly because they have the same valence electron configuration.
Electron configuration describes the arrangement of electrons in orbitals around the nucleus, following the aufbau principle and Hund's rule. The periodic table arranges elements so that groups (vertical columns) share the same valence electrons, predicting bonding and reactivity.
- 1↓1s1st shell, 2 electrons max
- 2↓2s, 2p2nd shell: 2s then 2p (6 total)
- 3↓3s, 3p3rd shell: 3s then 3p (8 total)
- 4↓4s4th shell: 4s (before 3d!)
- 5↓3d, 4pTransition metals (3d) then 4p
- 6Continue higher5s, 4d, 5p, 6s, 4f (lanthanides), 5d…
Step-by-step worked examples
Write the electron configuration for oxygen (Z=8).
Oxygen has 8 electrons to place. Fill in order: 1s² (2) → 2s² (2) → 2p⁴ (4) Configuration: 1s² 2s² 2p⁴ Alternatively: [He] 2s² 2p⁴
Nitrogen has 7 electrons. What is its valence configuration?
Nitrogen: 1s² 2s² 2p³ Valence shell (n=2): 2s² 2p³ (5 valence electrons) This explains N's tendency to gain 3 electrons (to reach octet)
Why are elements in the same group chemically similar?
Group 17 (halogens): F (2s² 2p⁵), Cl ([Ne] 3s² 3p⁵), Br ([Ar] 3d¹⁰ 4s² 4p⁵) All have 7 valence electrons (ns² np⁵ pattern) Identical valence config → identical bonding behavior
Flashcards
Quick quiz
Q1.What is the electron configuration of carbon (Z=6)?
Q2.Which orbital is filled after 4s?
Q3.What do elements in Group 1 (alkali metals) have in common?
Q4.How many valence electrons does chlorine (Cl, Z=17) have?
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Common mistakes
Thinking electrons fill orbitals in numerical order (1s, 2s, 3s… then back to 2p). — Correct: Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... Energy order, not shell order.
Confusing the number of electrons in an orbital with the orbital's capacity. — Correct: s orbitals hold max 2, p hold 6, d hold 10, f hold 14 (each can be empty).
Assuming all elements in a period (row) are similar. — Correct: Elements in a group (column) are similar. Periods show trends (size, ionization energy).
Ignoring valence electrons as 'less important' than core electrons. — Correct: Valence electrons determine chemistry; core electrons are mostly inert.
FAQ
What is the periodic table?
A table organizing all 118 known elements by atomic number and electron configuration. Groups (columns) share similar chemistry; periods (rows) show trends.
How do you write electron configuration?
Follow the aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… Each orbital gets superscript for electron count (e.g., 1s² 2s² 2p⁶).
What is a 'full' valence shell?
Usually 8 electrons (octet rule), making an atom stable. Noble gases (He, Ne, Ar…) have full valence shells and are unreactive.
Why are noble gases unreactive?
Noble gases have complete valence shells (full s and p orbitals). No driving force to gain, lose, or share electrons.